Survival of juvenile corals is vital for the recovery and resilience of reefs as global climate change and local stressors threaten the persistence of these ecosystems. However, newly settled coral recruits are at high risk for mortality due to their small size. Tissue fusion with neighboring recruits provides a pathway to rapidly increasing total colony size faster than would be achieved by growth alone. As self-recognition systems do not fully develop until several months post-settlement, fusion between newly settled juvenile colonies provides an opportunity for an increase in genetic diversity along with an increase in size. Although fusion provides advantages for survival across several species, it is unknown whether an increase in size or genetic diversity drives increases in survival. As ocean temperatures continue to increase, these potential benefits of fusion may provide ecological advantages during thermal stress events. In this study, We test these concepts by investigating the implications of fusion between newly settled Pocillopora acuta juveniles. We manipulated the genotypic richness and number of juveniles involved in tissue fusion to analyze influences on survival and growth in thermal stress. In ambient conditions, tissue fusion provided a significant survival advantage regardless of genotypic diversity, while negative competitive effects existed between closely settled, but non-fused, juveniles. In the presence of thermal stress, tissue fusion significantly delayed juvenile mortality and fusion between multiple genotypes provided an additional survival advantage. Delayed mortality as a result by tissue fusion and elevated genotypic diversity provides a window of opportunity for increased survival of fused juveniles. These results indicate that tissue fusion provides important ecological advantages in thermal stress for P. acuta, which are further enhanced by increased genetic diversity. Further research is required to determine energetic and genetic mechanisms that contribute to increases in survivorship as a result of tissue fusion.
Please see manuscript for detailed methodology
In this experiment, settled juvenile corals were secured on slides either as solitary individuals or in groups. Groups were composed of 2, 3, or 4 juveniles either from the same genotype or of multiple genotypes. See genetic analyzes for distinction of genotypes. These slides were then placed in an indoor tank system with light and flow-through seawater at ambient temperature for a period of 15 days, allowing them to grow and have the opportunity to fuse. This period is referred to as the “grow out period”. At the end of the grow-out period, we measured survivorship (alive or dead), growth (as number of polyps per juvenile), and fusion rate (fusion success or no fusion). Fusion was identified by continuous pigmentation across boundaries between juveniles with clear tissue connectivitiy. We then analyze the effect of genotypic richness and fusion on survival and growth. All responses in this experiment were measured at the level of the individual juvenile, not at the level of the slide community.
After the 15-day grow-out period, we exposed juveniles on slides to either a continuation of the ambient treatment (daily maximum temperature = 27.4 °C) or exposure to a high temperature (daily maximum temperature = 30.8 °C). At multiple points during this “stress period”, we measured survivorship (alive or dead), growth (numer of polyps per juvenile), and fusion persistence (fusion success or no fusion). We analyzed the effect of temperature, fusion (classified as the fusion status at the start of the stress period), and genotypic richness on survivorship and growth. Additionally, we analyzed the effect of temperature and genotypic richness on fusion persistence.
Experimental design schematic. Circles represent individual juveniles and colors represent genotypes.
Example of juveniles settled on slides as a group.